I'm a San Francisco-based Assistant Managing Editor with a focus on wealth. I edit mostly, but also write about how the richest get wealthy and how they spend their time and their money. My colleague Luisa Kroll at Forbes in New York and I oversee the massive reporting effort that goes into Forbes' annual World's Billionaires list and the Forbes 400 Richest Americans list. The former gets me to use my rusty Spanish and Portuguese. Tracking wealth is familiar territory: I ran the Forbes Billionaires list for five years back in the late '90s, when there were far fewer billionaires. Over 18 years my Forbes reporting has taken me to 17 countries on four continents, from the slums of Manila to palaces in Saudi Arabia and Mexico's presidential residence. Follow me on Twitter @KerryDolan My email: kdolan[at]forbes[dot] com Tips and story ideas welcome.

Methodology: How We Crunch The Numbers

More than 50 reporters in 16 countries worked on compiling our 25th anniversary World’s Billionaires rankings. Throughout the year our reporters meet with the list candidates and their handlers and interview employees, rivals, attorneys and securities analysts. We keep track of their moves: the deals they negotiate, the land they’re selling, the paintings they’re buying, the causes they give to. To estimate billionaires’ net worths we value individuals’ assets, including stakes in public and private companies, real estate, yachts, art and cash–and account for debt.

Not that we pretend to know what is listed on everyone’s private balance sheet, though some folks do provide that information. We do attempt to vet these numbers with all billionaires. Some cooperate, others don’t.

We have not included fortunes dispersed across large families (as in those of the Du Ponts) when individual net worths are below our minimum of $1 billion per name. But we do include wealth belonging to a member’s immediate relatives if the wealth can ultimately be traced to one living individual; in that case “& family” indicates that the number shown includes money belonging to more than one person.

Our estimates of public fortunes are a snapshot of wealth on Feb. 14, 2012, when we locked in stock prices and exchange rates from around the globe. Some on our list will become richer or poorer within weeks–even days–of publication. Privately held companies are valued by coupling estimates of revenues or profits with prevailing price-to-revenues or price-to-earnings ratios for similar public companies.

Acknowledgments

Forbes would like to thank the following people and companies for their help with research and expert advice.

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Not anymore. In 2011 Kamprad’s lawyers produced documents showing Forbes that he had irrevocably moved most of his stake in Ikea into a philanthropic foundation. That knocked billions off his net worth. He’s most recently worth $3 billion, ranked No. 377. See more here: http://www.forbes.com/profile/ingvar-kamprad/

I am looking for philanthropists who help individuals. Names and addresses where I could write to get assistance with mounting medical bills at Mayo Clinic, bills from the IRS, bills, bills, bills. My name is Susie Evans, 2486 East Harrison, Gilbert, AZ 85295. I don’t have a vehicle so I must rely on another person to get me to and from the Clinic; my insurance expires in November, my doctors have concluded that my stomach doesn’t empty and therefore, unless they can find alternatives I would be facing surgery which would mean I would have a colostomy bag for the rest of my life. I am 54 years old and need some help. I don’t want to go on food stamps or other social programs; I just want to be able to pay my bills and have a vehicle to take me to and from clinic visits. I am willing to work to repay anyone who can help me; I could clean houses, run errands, etc. to help you out for helping me out. My phone number is 406-855-8005 if you would like to talk to me. I can’t sleep at night worrying about how I am going to be able to pay my bills. I am not asking for a handout; I just need a hand for a little while to get me back to where I was before I became sick and then I can lend you a hand in return. Even if you can only afford $100.00 or less I would be EXTREMELY grateful and appreciative. I am a real person and this is NOT a scam; you can call me or write to me or get copies of my medical bills at Mayo Clinic. I would be willing to send you any documentation that you would require to insure you that I am completely telling you the truth. With sincere gratitude, Susie

Agreed! Privately held fortunes are harder to tease out than publicly-traded ones, and private wealth held by royal family members and in some cases political leaders is very difficult to document. For that reason we do not include in our annual FORBES World Billionaires List heads of state and royal family members unless we have specific documentation of assets.

I got into a debate tonight with a classmate around the margin for error in the estimation of number of billionaires if you were to also be able to count illegal enterprises, royalty/ heads of state and also undocumented fortunes / hidden billionaires.

If you were to estimate the total number of billionaires in the world, what do you expect the real range would be? +25%? +300%?